The Japanese written language contains three separate character sets. Simple Japanese characters representing phonetic syllables are represented by the hiragana and katakana character sets (together referred to as "kana"). Hiragana characters, which are characterized by a cursive style, are typically used for words native to Japan. Katakana characters, which are characterized by a more angular style, are typically used for words borrowed from other cultures, or for emphasis and sound effects. The third character set in Japanese is kanji. Kanji are the complex Japanese characters borrowed from the Chinese language. There are over 9000 kanji characters in the Japanese language. Approximately 4000 kanji are used on a semi-regular basis, while knowledge of 2000 kanji is generally required to read a newspaper or get around in Japan. One additional representation of written Japanese is the roman character or "romaji" character set. Romaji are romanized representations of Japanese words based on the sound of a character, and are typically used only to facilitate electronic input of Japanese characters in a computer, or to aid foreign students in the study of Japanese. The complexity of the Japanese written language poses several challenges for efficient text entry in computers, word processors, and other electronic devices.
Common methods of Japanese text entry for computers and like devices typically require the use of a standard Japanese character keyboard or a roman character keyboard which has been adapted for Japanese use. Both approaches, however, have several disadvantages. A disadvantage of the roman character keyboard approach is that often complex keystroke combinations must be mastered in order to generate the proper Japanese character, since an alphabetical arrangement of roman letters is not a natural order for entering Japanese text. A second disadvantage of this method is that the input romaji text must be converted to Japanese, thus imposing a processing overhead. A disadvantage of standard Japanese character keyboards is the high number of keys required to represent the basic Japanese character set.
The hiragana and katakana character sets each contain 46 base characters. Both sets of kana have identical pronunciations and rules of construction, only the shape of the characters are different to emphasize the different usage of the words. Some base kana characters are used in certain combinations and in conjunction with special symbols (called "nigori" and "maru") to produce voiced and aspirated variations of the basic syllables, thus resulting in a full character set for representing the approximately 120 different Japanese phonetic sounds. If a Japanese keyboard included separate keys for all of the voiced and aspirated variants of the basic syllables, the keyboard would need to contain at least 80 character keys. Such a large number of keys creates a crowded keyboard with keys which are often not easily discernible. If the nigori and maru symbol keys are included separately, the number of character keys can be reduced to 57 keys. However, to generate voiced or aspirated versions of a base character requires the user to enter two or more keystrokes for a single character. Another disadvantage of existing kana keyboards is that typically only one kana set (usually hiragana) may be input directly from the keyboard. Conversion to the other kana set requires a separate conversion operation, thus increasing processing overhead and user interaction. Thus, present methods of text entry through existing Japanese character keyboards involve keyboards which are overly complex and/or require a high degree of user interaction.
Many electronic devices are making increased use of on-screen or "virtual" keyboards to facilitate text entry. This is especially true with devices such as Personal Digital Assistants (PDA's), or computer or communications devices which feature minimal keyboards or keypads for text entry. On-screen keyboards are also used in computer-based communications systems in which a standard computer monitor and keyboard are not utilized. Examples of such systems include the Network Computer (NC) and similar dedicated network server systems. An on-screen keyboard is a picture of a keyboard with each key represented as a separate selectable graphic object. A cursor control device, such as an arrow button or similar directional control, is used to move a cursor or similar on-screen indicator to select a particular character. In a typical text entry situation, the cursor is moved to a desired key with the direction controls, and that key is then selected or entered using a separate button on the device. This process is then repeated for each letter in the word.
Existing systems which use a full Japanese character arrangement for on-screen keyboard implementations pose special problems because of the inherent display and text entry constraints of on-screen keyboards. For example, the implementation of a standard Japanese on-screen keyboard would may produce a crowded display of character keys which would be difficult to distinguish in a low-resolution or limited screen area.
It is therefore desirable to provide a system which allows the entry of Japanese kana characters into an electronic device with a keyboard which features a minimum number of Japanese characters, thereby reducing the display area occupied by the keyboard, minimizing required user input, and speeding the text entry process.